See 'The Mighty Anaconda' video as a good example of Frontier using the 3rd person view ...or pretty much ANY video they release actually.
Allow a 3rd person view only when:
1) In supercruise
2) During the hyperspace jump (which is really a form of loading screen)
3) When docking using the auto-docking computer
These three processes all use a 3rd person view in The Mighty Anaconda video and it looks great in doing so.
Not one of these three things offers any sort of combat or gameplay advantage (in 2 of them you don't even have control of the ship....) and for those that don't want to risk any break in immersion, they can simply not use the toggle switch on the keyboard to switch to this temporary 3rd person view.
At that point, I can't see any possible argument against allowing people to enjoy a small amount of 3rd person view other than stomping your feet and yelling, "no you can't have it because!"
Is it really hard to understand that access to an external view in a true first person game undermines the game being first person?
Frontier have been very clear the their design decision is all about "you" being "in" your ship. An external view operating outside of this first person mechanic is "you outside your ship."
This issue, as it relates to the design put forth by Frontier, has nothing to do with combat advantages or disadvantages.
Any external view in Elite: Dangerous, if Frontier are to keep in line with their stated reasons, must remain within the framework of "you in your ship."